1. Field of the Invention
The present invention generally relates to a scatterometer and, more particularly, to a reflective scatterometer.
2. Background of the Invention
The scatterometer plays a key role in measuring critical dimension in the semiconductor industry. As the critical dimension of semiconductor processing is shrinking down, it gets more and more difficult to measure overlay error between layers by conventional optical microscope or to measure the line width by scanning electron microscope (SEM). Therefore, the scatterometer system using of diffraction optics of fine lines and overlay structures by gratings has attracted tremendous attention.
Since the scatterometer exhibits excellent repeatability, reproducibility and optical non-destructive and high throughput measuring, the scatterometer has become important in semiconductor technology. Generally, the scatterometer can be divided into the spectrum scatterometer and the angular scatterometer.
In the spectrum scatterometer, the incoming beam is perpendicularly incident on a sample to measure the perpendicularly reflected zero-order diffracted beam. By setting up a measured reflectivity signature of the incoming beam with different wavelengths and comparing the measured reflectivity signature to the theoretically derived reflectivity signature, it can be determined whether the grating structure is defective. However, perpendicularity is not an optimal incident angle, which results in poor measuring sensitivity of the spectrum scatterometer. Moreover, the refractive index of a material at different wavelengths must be known before the theoretical reflectivity signature can be derived.
In the angular scatterometer, the angle of the incoming beam incident on a sample is changed and the zero-order diffracted beam is measured corresponding to different incident angles. By setting up a measured reflectivity signature of the incoming beam with different incident angles and comparing the measured reflectivity signature to the theoretically derived reflectivity signature, it can be determined whether the grating structure is defective. However, the angular scatterometer is usually complicated and is described with reference to accompanying drawings.
FIG. 1 is a structural diagram of a conventional reflective scatterometer, which is disclosed in U.S. Pat. No. 5,703,692. Referring to FIG. 1, the conventional reflective scatterometer 100 is capable of measuring the grating structure (periodic structure) on a sample 50. The reflective scatterometer 100 comprises a light source 110, a rotating block 120, a beam splitter 130, a focusing lens 140 and a detector 150. The light source 110 generates a collimated beam 112, which is incident on the beam splitter 130 after passing through the rotating block 120. The sample 50 is disposed at the focal point of the focusing lens 140. The collimated beam 112 is focused on the sample 50 after being reflected by the beam splitter 130 and passing through focusing lens 140 so as to generate a diffracted beam 114, which is zero-order diffracted.
Accordingly, the diffracted beam 114 is received by the detector 150 after passing through the focusing lens 140 and the beam splitter 130 so that the intensity of the diffracted beam 114 can be measured. Moreover, the rotating angle of the rotating block 120 is adjusted to translate the collimated beam 112 to change the incident angle of the collimated beam 112 on the sample 50. Therefore, the reflectivity can be measured corresponding to different incident angles to determine whether the sample 50 is defective.
FIG. 2 is a structural diagram of another conventional reflective scatterometer, which is disclosed in U.S. Pat. No. 6,987,568. Referring to FIG. 2, the conventional reflective scatterometer 200 is capable of measuring the grating structure on a sample 50. The reflective scatterometer 200 comprises a light source 210, an aperture 220, a beam splitter 230, a parabolic reflector 240 and a detector 250. The parabolic reflector 240 has an optical axis 242. The sample 50 is disposed at the focal point of the parabolic reflector 240 and the normal direction 52 of the sample 50 is perpendicular to the optical axis 242.
Accordingly, a collimated beam 212 generated by the light source 210 is reflected by the beam splitter 230 onto the parabolic reflector 240 after passing through the aperture 220. The collimated beam 212 is then reflected by the parabolic reflector 240 onto the sample 50 to generate a diffracted beam 214, which is zero-order diffracted. The diffracted beam 214 is reflected by the parabolic reflector 240 to pass through the beam splitter 230 to be received by the detector 250 so that the intensity of the diffracted beam 214 can be measured. Moreover, the location of the aperture 220 is adjusted to translate the collimated beam 212 to change the incident angle of the collimated beam 212 on the sample 50. Therefore, the reflectivity can be measured corresponding to different incident angles to determine whether the sample 50 is defective.